Advanced composites are increasingly used as reinforcing components in aircraft, automotive, and sporting goods applications. Typically, these composites comprise a strengthening component, such as carbon fibers, embedded in a thermosetting resin matrix. Components fabricated from such carbon fiber reinforced resin composites are produced by impregnating oriented carbon fibers (in the form of woven carbon cloth, or continuous carbon filaments) with thermosetting resins, and arranging the filaments of carbon fibers to form prepregs. Generally, prepregs include a paper backing onto which the fiber reinforcement is laid, and the selected resin is then forced into the fibers.
Thermosetting resins, which normally include a latent curing agent that is activated by increasing the temperature of the resin over a minimum cure temperature, are often preferred over two part resin systems that cure very quickly once the two parts are mixed. This preference arises from the ease of handling of thermosetting prepregs during the manufacture of components. Thermosetting prepregs can be produced in quantity with consistent properties, and stored at cold temperatures for a considerable length of time before use. Most of these thermosetting prepregs are provided as large rolls of material that include the paper backing and the epoxy impregnated fibers. To use this material, the desired portion is simply cut from the roll. In contrast, two part resin mixtures, which admittedly often have desirable properties, such as a rapid cure time, must be used immediately after being produced. Thus, component manufacturers can only produce small batches of composite material, and must then use each batch immediately after it is mixed. Furthermore, the chance that a bad batch of composite material will be produced on a manufacturer's production line is considerably higher than in a chemical plant where the focus is on the single task of producing a prepreg. The ease of use of prepregs is such that even though the curing performance of two part resin systems is generally better than that of thermosetting resins, prepregs are almost universally preferred for fabricating composite reinforcing components.
To fabricate a reinforcing component from prepregs, manufacturers generally apply multi-layer laminates of these prepregs over existing molds. To generate a rod or shaft, the prepreg is wound around a mandrel. Once a sufficient number of laminations have been achieved, the mold is heated to the cure temperature required to activate the latent curing agent in the thermosetting resin, using an oven or autoclave. Generally, a higher temperature results in a shorter cure time, while a lower temperature requires a longer cure time.
Suitable thermosetting epoxy resins generally have more that one epoxide group per molecule. In addition to the latent curing agent, which is often a functional amine, state-of-the-art epoxy matrix resin systems used in advanced composites often employ a catalyst, which helps to reduce cure times. It should be noted that cure times are important to end users of prepregs, especially when prepregs are used in conjunction with molds. Molds can range from the simple to the complex, depending on the component being produced. To ensure a high level of production quality, a manufacturer must spend considerable time and effort to ensure that each mold is identical. Furthermore, each mold must be able to be heated to activate the prepreg, thus increasing the equipment required for each mold. For example, in a golf shaft manufacturing process, commonly used prepregs need to be cured for more than 20 minutes at 150° C. in order to be removed from the mold or mandrel without changing shape. If a manufacturer can obtain a prepreg with a cure time which is one half of a presently used product, then that the manufacturer can double production without providing additional molds. It would therefore be desirable to provide an advanced epoxy resin system that substantially reduces cure time, to enable manufacturers to increase production without providing additional molds.
Because prepregs are often used to form reinforcing components, such as parts for airplanes, the resulting components must meet high quality standards. It is desirable that any reduction in cure time not negatively affect the physical properties, such as tensile strength, of such composite components.
In addition to preferring prepregs that have shorter cure times, composite component manufacturers also desire prepregs that cure at lower temperatures, particularly manufacturers who fabricate large scale composite components, such as those that might be employed in aviation or marine applications (boat hulls, for example). Low temperature curing prepregs are desirable for manufacture of large parts because such low temperatures require less sophisticated heating systems, and much reduced energy costs, which can be significant for large scale parts. Note that one major manufacturer of prepregs, Hexcel Corporation of Dublin, Calif., currently offers a low temperature curing prepreg (M34™), which cures at 65° C. (for 16 hours) or 75° C. for 8 hours. It would be desirable to provide an advanced epoxy resin system that substantially reduces cure time below that of currently available prepregs, particularly at low cure temperatures.
Many different types of epoxy resins systems are known in the art. Different combinations of epoxy resins, curing agents, and catalysts (also known as accelerators) have been formulated. A balance of desirable properties for prepregs include the following: (1) a tacky, dough-like consistency prior to curing; (2) low reactivity at room temperature; and, (3) a high degree of cure after heating for no more than 2 hours at no more than 180° C. As noted above, the provision of a prepreg with a reduced cure time will offer component manufacturers significant efficiency advantages. Accordingly, there is an ongoing effort within the prepreg industry to produce a prepreg that has the desired consistency and low reactivity at room temperature, yet also exhibits reduced cure time at relatively low temperatures.
While certainly not an exhaustive compilation, the following patents provide examples of thermosetting resin compositions known in the art. International Patent Publication No. WO 99/36484 describes a composite system that includes an epoxy resin having two or more epoxide groups per molecule, a latent hardener and at least one solid organic acid that is substantially insoluble in the resin formulation. U.S. Pat. No. 3,759,914 (Simms) discloses an epoxy resin formulation including a polyepoxide having a plurality of epoxide groups, a latent amine curing agent and an accelerator having a defined formula. U.S. Pat. No. 3,386,956 (Nawakowski) describes an epoxy resin formulation including a primary curing agent selected from a range of bis- and polyureas, and a promoter selected from the following four compounds: dicyandiamide (DICY), stearic hydrazide, succinimide and cyanoacetamide. The function of the promoters in Nawakowski's formulations are to increase the cure rate at low temperatures (i.e. less than 187° F.). The bis-ureas described include 2,4-di (N,N-dimethylureido)toluene, also known as 2,4-toluene bis dimethyl urea.
A similar epoxy formulation is disclosed in U.S. Pat. No. 3,386,956 (Harrison), which employs a polyamine curing agent and a phenyl urea based accelerator (see also U.S. Pat. No. 3,988,257 for related methods). U.S. Pat. No. 3,956,237 (Doorakian) describes an epoxy resin formulation including a latent amine curing agent and a latent accelerator. A number of latent accelerators are disclosed, including a specific blend of different isomers of toluene bis dimethyl urea. U.S. Pat. No. 4,569,956 discloses a rapid, low temperature curing epoxy resin adhesive composition comprising a polyepoxide, a catalytic amount of HBF4, a finely divided filler (preferably an acidic filler) and, optionally, a polyalkylene ether glycol. Yet another epoxy formulation is disclosed in U.S. Pat. No. 4,783,518 (Goel), which teaches a rapid curing epoxy composition including a polyepoxide, a latent amine curing agent, a novel thiocyanate salt of the reaction product of an alkylene polyamine (such as ethylene diamine) and a bicyclic amide acetal. U.S. Pat. No. 5,407,978 (Bymark) describes an epoxy formulation which includes a dihydric bisphenol curing agent and a immidazole based accelerator to increase the cure rate. As a final example, U.S. Pat. No. 5,599,629 (Gardner) describes an epoxy resin formulation including a resin with at least three epoxide groups per molecule and a specific aromatic amine latent curing agent, the aforementioned formulation being specifically employed to produce prepregs.
While the above-cited references all assert that a functional formulation having desirable properties is achieved, composite component manufacturers still desire a prepreg material having faster cure times, and/or lower cure temperatures. It would be desirable to provide an epoxy formulation differing from those described in the prior art, that is adaptable to being employed as a prepreg, and which provides shorter cure times and lower cure temperatures than existing prepregs provide.
It should be noted that several different methods can be used to fabricate prepregs, including a solventless, hot melt impregnation method, and a solvent method. In a typical hot melt impregnation process, continuous sheets of resin matrix film supported by release paper are impregnated into fiber sheets under heat, pressure, and tension. The matrix has to have a certain viscosity at impregnation temperature so that the resin can wet-up the fiber. Furthermore, specific tack, drape, and shelf-life characteristics are required when utilizing the hot melt method. In contrast, a solvent-diluting impregnation method does not have such strict requirements. However, a superior prepreg is often achieved by the hot melt method, because micro-voids, caused by off gassing of volatile solvent, are often observed in prepregs prepared by the solvent-diluting impregnation method. It would be desirable to provide an advanced epoxy resin system adaptable to be employed to produce a prepreg, which substantially reduces cure time, that can be used with either the hot melt impregnation method or the solvent based impregnation method.
In addition, it should be noted that the time required for a prepreg to cure is not always the limiting factor determining when the cured prepreg can be removed from a mold. For example, a commonly utilized prepreg material is produced from an epoxy formulation including epoxy resin A (a diglycidyl ether of bisphenol A having an epoxide equivalent weight (EEW) of 176), epoxy resin B (a diglycidyl ether of bisphenol A having an EEW of 1200-1400), a thermoplastic additive (PVF powder), a DICY curing agent, and a catalyst (3,4-dichlorophenyl-N,N-dimethylurea, available as DYHARD UR200™, made by SKW Trostberg). Depending on the specific proportions of the above ingredients employed, it is possible to produce a prepreg whose glass transition temperature (Tg) is significantly lower (20° C.) than the optimal cure temperature. For instance, manufacturers of composite shafts frequently employ mold temperatures of 300° F.-310° F. (147° C.-153° C.) to obtain rapid cure times. However, such temperatures are generally above the Tg of the resin component, and while the resin component is fully cured, it will be too soft to be removed from the mold. In such cases, a manufacturer must cool the mold below the Tg before removing the cured component from the mold. This cooling step is an additional, undesirable step, which increases the time required to produce a component, lowers the number of components that can be produced by a mold during a work cycle, and undesirably increases costs. It would therefore be desirable to provide an epoxy resin formulation, suitable for making prepregs, that exhibits reduced cure times, and having a cure temperature that is either less than or about (within 10° C. of) the Tg of the cured prepreg material. While high temperature curing resin systems are known, which have a cure temperature that is less than the Tg of the cured resin, such resin systems require long (in excess of two hours) cure times. The prior art does not teach or suggest a rapid curing epoxy resin formulation whose cure temperature is sufficiently close to the Tg of the cured resin so that cooling of the mold is not required.
It would further be desirable to provide an epoxy resin formulation that is not only suitable for making prepregs, but which can also be beneficially employed to fabricate thermosetting resin adhesive film products by coating a relatively thin layer of resin onto a backing material, such as paper or film. Such a thermosetting resin adhesive film product will desirably have good workability at room temperature, and be activated by exposure to an appropriate temperature condition.